Autism spectrum disorders (ASDs) are a group of 5 neuro developmental conditions (autism, Asperger syndrome, pervasive developmental disorder not otherwise specified [PDD-NOS], Rett syndrome, and disintegrative childhood disorder).1 Once thought to be rare, the incidence of these disorders is now estimated to be 1 in 150 children in the general population.2 Furthermore, the number of recognized cases has increased markedly in recent years.

Many factors have contributed to the increasing numbers of persons with ASD. These include expanded diagnostic criteria, more diagnostic groups, extended evaluations that range over an entire life span, better and more comprehensive assessment methods, more and better trained professionals, and better funding for research and screening.3 Croen and associates4 found similar rates of developmental disability overall. The rise in ASD diagnoses has paralleled a decline in primary diagnoses of intellectual disability.

ASD does not inoculate persons from the range of psychiatric conditions that affect others in society. We follow the lead of Gilberg and Billstedt5 who describe comorbidities as overlapping disorders. A descriptive definition versus one that implies a direct or indirect link is important, because it is also possible that comorbid disorders occur by chance without an overlapping cause.

Psychiatric disorders are distinct from challenging behaviors that co-occur at high rates with ASD, along with conditions such as intellectual disability and epilepsy. Some forms of psychopathology are particularly common in ASD including atten- tion-deficit/hyperactivity disorder (ADHD), psychosis, depression, anxiety, and obsessive-compulsive disorder (OCD).6,7 Consequently, the clinician needs to:

• Determine whether the patient has ASD. If this diagnosis is made, the type of ASD needs to be pinpointed along with the severity of specific symptoms, as well as the severity of the overall disorder.• Identify challenging behaviors and determine whether they are environmentally maintained or if they are linked to ASD or co-occurring conditions.8• Assess whether comorbid intellectual disability is present.• Consider the possibility of comorbid psychopathology.

While variability in level of overlap in these disorders is debatable, experts agree that overlap is considerable.

The specifics on how to address each of these issues follow, using the best available evidence-based practices. In all instances, the use of a reliable and valid test, clinical observation, school or home observation or report, clinical history, and clinical consensus constitutes the gold standard in inpatient settings.

Diagnosing ASDDevelopmental factors are in play relative to this spectrum of diagnoses. Rett syndrome, which is quite rare, can be reliably evaluated via genetic testing; it can be identified earlier than other forms of ASD. The severity of symptoms and course in Rett syndrome vary over time. Thus, additional behaviorally based assessments of symptoms should be made over time. Autism (in about half of cases) and disintegrative childhood disorder (in all cases) involve marked loss of previously acquired skills at approximately 18 to 30 months of age. For the remainder of children with autism, diagnosis as young as 18 months may be possible, although some debate exists regarding the exact age cut-off point that will produce a reliable and valid diagnosis.9

Filipek and colleagues10 report on the number of unrecognized cases of autism before school age and the need for more intensive screening efforts. They emphasize that screening is particularly important for children at risk for any type of atypical development. However, regression at approximately age 2 and the heterogeneity of symptoms make this a daunting task.

No consensus exists on how early PDD-NOS or Asperger syndrome can be diagnosed. However, because PDD-NOS has milder symptoms than autism, it may be more difficult to identify in young children. Children with Asperger syndrome have a normal or higher than normal IQ; consequently, this disorder cannot be diagnosed as early as other ASDs.

There has been ongoing debate about whether Asperger syndrome is a separate diagnosis from high-functioning autism (HFA).11 We recommend the use of a scale that is designed to aid in diagnosing suspected Asperger syndrome.12 Thus, for early identification, autism and PDD-NOS will be the disorders screened for by the clinician in most instances. The Table presents scales/tests with established psychometrics that can facilitate diagnosis. Use of at least one of these measures as part of the clinical process is advisable.